Commercial vehicles, such as Class 8 trucks, typically employ a non-synchronous transmission to transmit torque generated by the engine to the vehicle's drive wheels. A non-synchronous transmission is a form of manual transmission based on gears that do not use sychronizing mechanisms or devices, referred to as dog clutches, cone clutches, dog collars, locking rings, etc., to assist in the meshing of gears when changing gears. Because the gear boxes are engineered without sychronizing mechanisms or devices, the non-synchronous transmission requires an understanding of gear range, torque, engine power, range selectors, multi-functional clutches, and shifter functions. Shifting involves techniques known as “double clutching” and “float-shifting.”
Non-synchronous transmissions are typically paired with a driver-operated clutch and a movable shift lever for selecting a reverse gear and a number of forward gears. To shift the transmission from one forward gear to another forward gear, the operator “floats” the transmission in and out of gear by either dis-engaging the clutch (i.e., separating the clutch plates from the flywheel) by pressing the clutch pedal only approximately half way to the floor or speed matching the engine speed and the output shaft speed in a technique known as “float shifting.”
Non-synchronous transmissions often have a mechanism for slowing down or stopping the input shaft and/or idle gears from turning to allow initial forward or reverse gear engagement. In commercial motor vehicles, this mechanism is called a clutch brake. The clutch brake is typically a circular disc with a friction surface that is splined to the input shaft of the transmission between the release or throw-out bearing and a transmission cap or brake surface, and is engaged by depressing the clutch all the way to the floor. The clutch brake is only intended to be used when engaging the transmission from a standstill, either shifting the vehicle into reverse or one of the starting gears, such as 1st gear. In some instances, the clutch brake can prevent shifting into gear until the clutch is lifted a few inches off the floor. Moreover, engaging the clutch brake while the vehicle and the transmission gears are moving can cause severe damage to the clutch.